Heavy-duty trucks, buses and other large vehicles are equipped with pneumatic or hydraulic brake actuating systems. The brake actuating system applies pneumatic or hydraulic fluid under pressure to a service chamber of a brake actuator to move a diaphragm or piston to actuate the brake system.
The brake actuator system further includes a parking brake actuator including one or more coil springs which actuates the brake when the fluid pressure is released, serving as a parking brake, and which will actuate the braking system in the event that the pneumatic or hydraulic system of the vehicle fails, serving as an emergency braking system preventing a runaway vehicle. The brake actuating system of this invention is particularly directed to vehicle parking brake actuators.
There are several types of parking brake actuators, including piston-type parking brake actuators and diaphragm-type parking brake actuators. In the diaphragm-type parking brake actuators, the power spring is maintained in a compressed state by pneumatic pressure acting against a flexible diaphragm. In piston-type parking brake actuators, the piston includes a head portion which sealingly engages an internal surface of the brake actuator housing and pneumatic or hydraulic pressure acting against the piston head maintains the power spring in a compressed state. The present invention is particularly, but not exclusively, adapted for piston-type parking brake actuators including hydraulic brake actuators. Driveline parking brake actuators generally include a cable connected to the distal end of the piston of the brake actuator, wherein movement of the piston by the power spring tensions the cable and actuates the vehicle parking brake.
There are several problems associated with the design of a driveline parking brake actuator. First, the spring rate or force provided by the spring should be substantially constant over the stroke of the piston. Second, the spring rate must be within the parameters required for the application, typically 55 to 65 lbs/in. Third, side loads should not be transferred to the piston to avoid wear or binding of the brake actuator which may reduce the life of the brake actuator or cause failure. Finally, the length verses diameter ratio must be within defined parameters to avoid buckling. It is also desirable to reduce the size and volume of the driveline parking brake actuator to increase its application. Further, it would be desirable to provide a “soft” or low spring rate which is constant over the stroke of the piston having the required spring rate. These objects are achieved with the driveline vehicle parking brake actuator of this invention as set forth below.